A vaccine-derived version of poliovirus has repeatedly surfaced in London sewage over the past several months, suggesting there may be cryptic, or hidden, spread among some unvaccinated people, UK health officials announced Wednesday.
No cases of polio have been reported so far, nor have there been any identified cases of paralysis. But sewage sampling in one London treatment plant has repeatedly detected closely related vaccine-derived polioviruses between February and May. This suggests “it is likely there has been some spread between closely-linked individuals in North and East London and that they are now shedding the type 2 poliovirus strain in their feces,” the UK Health Security Agency (UKHSA) said.
Though the current situation raises alarm, the agency notes that it’s otherwise common to see a small number of vaccine-like polioviruses pop up in sewage from time to time, usually from people who have recently been vaccinated out of the country. This is because many countries use oral polio vaccines that include weakened (attenuated) polioviruses, which can still replicate in the intestines and thus be present in stool. They can also spread to others via poor hygiene and sanitation (i.e. unwashed hands and food or water contaminated by sewage), which can become very concerning amid poor vaccination rates.
How and why this happens
Briefly, there are two types of polio vaccines: the attenuated oral vaccines and inactivated vaccines. Many high-income countries that are considered polio-free—including the UK and the US—use the inactivated vaccines, which do not have viruses capable of replicating or spreading. These vaccines are highly effective at preventing paralytic polio, but they do not produce high levels of local immune responses in the gut. So, if a vaccinated person encounters wild poliovirus, the virus may still be able to replicate in their gut and spread. In areas affected by wild polio outbreaks, this means that the virus can continue spreading.
Oral polio vaccines, on the other hand, can not only prevent paralytic polio, they can also produce strong local immune responses in the gut that block the virus from replicating there, thus disrupting spread. These vaccines can also be more than five-times cheaper than the inactivated kind. For all of these reasons, oral polio vaccines are the predominant vaccines used in the long, drawn-out battle to eradicate wild polio. Currently, wild polio is still found in Afghanistan and Pakistan, and Malawi and Mozambique have each recently reported single cases.
But, one of the downsides to oral polio vaccines is that vaccinated people can shed the attenuated vaccine virus in their stool for several weeks after vaccination. If this happens in a community with poor sanitation, hygiene, and low vaccination coverage, the vaccine virus can spread from person to person. Over time, as the vaccine virus spreads to more people, it can pick up mutations that make it more like wild-type polio, allowing it to regain the ability to cause disease and, in rare instances, paralysis in unvaccinated people. At this point, the mutated vaccine virus gets dubbed “vaccine-derived poliovirus” or VDPV. Recently, VDPV cases have been reported from several African countries and Israel.